1) Field of the Invention
This invention relates to a ball screw device with internal motors, which is suitable for use as actuators in machine tools, scalar robots or other industrial equipment.
2) Description of the Related Art
Conventional ball screw devices equipped with both a ball screw nut and a ball spline nut on a single common shaft are disclosed, for example, in U.S. Pat. No. 4,939,946 issued on Jul. 10, 1990. In this conventional ball screw device, a helical ball screw groove and linear, axial ball spline grooves are both formed on an outer circumference of a single shaft. A ball screw nut is fitted on the shaft via a number of balls received in the ball screw groove, while a ball spline nut is similarly fitted on the shaft via a number of balls received in the ball spline grooves. Both the nuts are rotatably assembled on a common housing, each by means of a combination of support bearings, whereby a ball screw unit is constructed. Both the nuts are connected to external drive motors by way of timing belts, respectively. The individual nuts are independently rotated or driven by the corresponding external drive motors, whereby the shaft is caused to undergo a reciprocating or rotating motion or a combined reciprocating and rotating motion.
The above-described conventional ball screw is of the construction that the ball screw unit and its drive units are discrete and are connected by the belts as power transmitting means. It is, therefore, accompanied by the problems that a limitation is imposed on its size reduction and further limitations are imposed on its application purpose and range in view of its installation space and position, its combination with an associated machine, or the like. Further, its power transmitting elements involve a play such as slipping of the belts, leading to the problem that the positioning accuracy is limited. In the case of a clean room specification, it is necessary to hermetically seal not only the ball screw nut and ball spline nut but also the power transmitting means and even the drive motors within a dustproof casing so that dust can be prevented from spreading into the clean room. This has resulted in the problem that the overall size of the device naturally becomes greater.